Recently, both computer-to-plate and on-press digital imaging of offset plates have become widely spread. The Imaging may be performed by a large number of writing beams to increase the image exposure speed.
A conventional multi-beam scanning apparatus may comprise a rotating drum and a scanning head having a plurality of laser diodes movable parallel to the axis of the drum. An exposure medium, such as an offset plate, may be mounted around the outer peripheral surface of the drum.
The drum may rotate at a fixed rotational speed indicating the fast scanning direction. Concurrently, the scanning head may move at a fixed speed, synchronized with the rotation of the drum in the axial direction of the drum indicating the slow scanning direction. On each rotation of the drum, the scanning head progresses a distance W. W is termed the scanning swath width. As a result of the two movements, each laser diode spirally scans the outer peripheral of the drum, forming a scanning line at an angle xcex1 to a direction perpendicular to the axis of the drum.
The resulting image exposed in this helical mode is not rectangular but rather slanted at angle xcex1, forming a parallelogram. The helical angle xcex1 is a function of the width W and the drum circumference DC, where       tan    ⁢          xe2x80x83        ⁢    α    =            W              D        ⁢                  xe2x80x83                ⁢        C              .  
The number of exposure beams N and the resolution R define tho scanning swath width as W=N/R, hence xcex1=arctangent [(NIR)IDC]. The helical angle xcex1 increases as the number of beams N increases. For example, for 32 beams at a resolution of 2540 dots per inch (100 dots/mm) and a drum circumference of 640 mm, the helical angle is: a=arctangent [(N/R)/DC] =arctangent [(32/100)/640]=0.0005 radians. If the number of beams is increased to 48 beams the helical angle xcex1 is increased to 0.00075 radians. In order to ensure the rectangular form of an image exposed in a helical mode, it is necessary to compensate for the deficiency described hereinabove.
Some existing methods for image distortion correction include inclining the scanning head itself at an angle equal to the helical angle, feeding the exposure media at the helical angle or advancing the recording start position for the scanning lines as scanning progresses. These methods may result in a rectangular image, which is inclined with respect to the central axis of the imaging cylinder at an inclination angle xcex1. This solution may be acceptable to some extent for computer-to-plate devices. However, it is generally unacceptable for automated computer-to-press imaging systems.
There is a need to provide a simple and flexible method of image distortion correction that would support the exposure with a varying number of beams and at variable exposure resolutions, without the need to make any mechanical adjustments on the press.